Most of us know that you may code an MI as a secondary diagnosis if it has occurred four weeks or less from the date of the encounter.

To be specific, the ICD-10-CM Official Guidelines (I.C.9.e.1, page 51.) state:
...For encounters occurring while the myocardial infarction is equal to, or less than, four weeks old, including transfers to another acute setting or a postacute setting, and the myocardial infarction meets the definition for “other diagnoses” (see Section III, Reporting Additional Diagnoses), codes from category I21 may continue to be reported.
For encounters after the 4 week time frame and the patient is still receiving care related to the myocardial infarction, the appropriate aftercare code should be assigned, rather than a code from category I21.
For old or healed myocardial infarctions not requiring further care, code I25.2, Old myocardial infarction, may be assigned.”
But before making the recommendation, be certain that the MI qualifies in some way as a secondary diagnosis. Did it require evaluation? Consume resources?

But what if the patient has a second MI within four weeks of the first? Here continued reading of the guidelines (pp. 51-52) provides an answer:
Subsequent acute myocardial infarction
A code from category I22, Subsequent ST elevation (STEMI) and non-ST elevation (NSTEMI) myocardial infarction, is to be used when a patient who has suffered a type 1 or unspecified AMI has a new AMI within the 4 week time frame of the initial AMI. A code from category I22 must be used in conjunction with a code from category I21. The sequencing of the I22 and I21 codes depends on the circumstances of the encounter.
Do not assign code I22 for subsequent myocardial infarctions other than type 1 or unspecified. For subsequent type 2 AMI assign only code I21.A1. For subsequent type 4 or type 5 AMI, assign only code I21.A9.

If a subsequent myocardial infarction of one type occurs within 4 weeks of a myocardial infarction of a different type, assign the appropriate codes from category I21 to identify each type. Do not assign a code from I22. Codes from category I22 should only be assigned if both the initial and subsequent myocardial infarctions are type 1 or unspecified.

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